1. Field of the Invention
The instant invention relates generally to a method and apparatus for calibrating a densitometer based sensor used to measure the density of shot within a given field. Obtaining the density of shot affords the ability to calculate the mass flow rate of shot in the system and the velocity of the shot leaving the system. If precise density readings are obtained, an operator may properly adjust the system to ensure that a required amount of shot leaves the system at a required velocity.
The instant invention therefore provides a quick and inexpensive way to determine the calibration constant of a densitometer based sensor. More specifically, the instant invention provides a way to determine the calibration constant of a densitometer based sensor used to measure the density of shot within a portion of a shot peening apparatus. The instant invention therefore ensures that a shot peening system meters are indicating the true operating parameters.
2. Description of the Related Art
The use of shot peening is relatively well known. In particular, a stream of shot (i.e. particles) is directed at the surface at a high velocity. The shot is directed at a work piece so as to cause plastic deformation of the surface of the work piece, often a metal surface. The shot peening is often used to increase fatigue strength, although the process may be applied for other purposes.
Various shot peening devices and techniques have been developed over the years. Shot peening systems, generally, have (or can be readily equipped with) mass flow controllers. Such controllers are used to control the flow of shot to the shot peening gun. One common type of mass flow controller for use with shot made from ferromagnetic material has an electromagnet which is pulsed in order to allow passage of a metered amount of shot into a shot peening gun. This and other common types of mass flow controller use internal feedback from a densitometer based sensor to stabilize the mass flow rate (i.e. the amount of shot metered in a given time). A control may be used to set the mass flow rate to a desired value. A display may be used to indicate the flow rate.
The quality of work accomplished by the shot peening apparatus depends on the velocity of the shot as well as the mass flow rate thereof. U.S. Pat. No. 4,805,429 to the instant inventor and assigned to the same assignee as the instant invention, discloses the extent of the importance of such velocity in a shot peening operation. Problems can arise when the sensor monitoring the mass flow rate or the nozzle velocity are not properly calibrated. Under-peening can lead to gaps in the surface compressive layer while over-peening can lead to embrittlement and damage. Similarly, the correct shot velocity assures that the depth of the compressive layer is maintained. Thus, it is vital to the shot peening process that the shot flow rate and velocity be accurately measured and controlled at all times.
In U.S. Pat. Nos. 4,873,855 and 5,176,018, assigned to the instant assignee, the instant inventor disclosed magnetic and densitometer based capacitance sensors, respectively, and each is hereby incorporated by reference. Each said disclosure also describes in detail how the sensors are used in a shot peening system to indicate the shot mass flow rate within the system and the shot velocity as the shot leaves the system.
U.S. Pat. No. 5,226,331 to R. A. Thompson et al and assigned to the same assignee as the instant invention discloses a prior apparatus for measuring the velocity and density of shot leaving the nozzle of a shot peening system. Prior to the instant invention, said prior apparatus was used to measure the velocity and density of shot leaving a shot peening system. The densitometer based sensors of the shot peening system were then calibrated to the readings taken.
The principle object of the instant invention is to provide a simpler and inexpensive means which is accurate and reliable for calibrating and insuring the accuracy of densitometer based shot peening sensors.
A further object of the instant invention is to provide a simpler and inexpensive method which is accurate and reliable for calibrating and insuring the accuracy of densitometer based shot peening sensors.